This is classic Hebrew parallelism. The two meanings are identical, but the B-parts of the cola (lie - repent) is different. Note that "God" from the first colon is not repeated, and hence the author may have chosen a longer term in the second colon to keep the cola equally heavy (in terms of syllables).
The same structure is also present in the second half of the verse, where אמר and דבר are also largely synonymous.
In general, son-of-X is a very common Hebrew way to indicate peoples, and also mankind. The X does not always bring associations of the proper name. When somebody says he is an Athenian, this does not immediately cause us to think about the Greek goddess Athena, and similarly when somebody is a son-of-Adam we should not take this to refer to the Adam from Genesis.
The word for repent, נחם, is a rather general root which indicates regret over plans made previously. It does not primarily indicate a promise to act differently in the future (which is what English "repent" suggests), but focuses on the past. The NIV provides a more fitting translation here with "change his mind". This is more in line with the parallelism as well: while humans lie or say something but change their mind, God remains true to his word. This is also what is reflected in the second half of the verse (which however moves the focus to the things God will do). Lastly also note that נחם is in the hitpael, which does not mean "repent" in the English sense anywhere; this translation is largely based on the sense of the niphal, while the hitpael of this root focuses more on the inner state of the subject.
Since the Bible is a product of many different people from very different times and cultures, I don't see a problem with the apparent 'contradiction' with Exod. 32:10,14 or other instances (Deut. 32:36; Ps. 135:35). But even if one wants to resolve all such apparent contradictions this one is not particularly hard: Num. 23:19 need not be read as a general rule. You may know of the distinction between the negation particles לא (which marks general rules) and אל (which marks particular rules). Here, we have לא thrice, but not in that sense: the first is not a command-לא; the last two are dominated by a conditional (אמר and דבר) which makes them non-general. The rules here are expressed by the yiqtol conjugation, which marks verbal modality as a rhetoric device: "would he lie/repent?"